Throughout the following specification and in the claims appended hereto the term "pre-cured" will be employed to describe pre-formed tire tread stock which is to be attached to the perimeters of buffed tire casings even though some preformed tread stocks are not completely cured, where originally fabricated but are only at least partially cured, while others are completely cured. The actual degree of pre-curing of the tread stocks embodying the invention and the method for the use thereof in re-capping tires are not critical in as much as the method when carried out completes any curing necessary to produce a completely cured, re-capped tire.
The use of pre-cured treads for recapping or retreading vehicle tires and some methods by which the pre-cured treads are attached to the casings of the vehicle tires are well known in the art. Among the more successful ones of these methods are those which employ curing envelopes. The pre-cured tread is stretched around and attached ("stitched") to the peripheral surface of a suitably buffed tire casing. An annular curing envelope, usually made of rubber, is stretched around the exterior surface of the tread. The external surface of the envelope is then exposed to fluid under pressure, for example, hot steam, while the air between the envelope and the tread is vented to atmosphere, to press the tread stock and an intermediate layer of unvulcanized rubber ("cushion gum") firmly against the surface of the tire casing and to hold the tread stock and cushion gum against that surface while the cushion gum (and the tread stock, if not fully pre-cured) is vulcanized by the heat and pressure ("cured") to firmly adhere the tread to the surface of the casing.
A problem exists in methods using a curing envelope in the conventional fashion, where the tread cross-section includes tapered edges or wings that are designed to curl around what might be called the shoulders of the tire casing itself. Even though the curing envelopes utilized in such methods have been extended laterally to go far around casing shoulders and even all the way around to the casing rim beads, the feather edge of a tapered wing frequently is not adequately adhered to the tire casing in methods utilizing curing envelopes.
The tendency of the tapered wings of the tread stock to lift away from the surface of the tire casing at the shoulders is greater, of course, when the configuration of the tread stock includes large lugs or raised sections of rubber which extend across the tread stock and must be bent when the tread stock is laterally curved to conform its under surface to the surface of the tire casing.
Another serious problem exists with this method which is difficult to overcome, particularly with complex tread designs having narrow grooves in their outer or "earth contacting" surfaces. Attempts have been made to force the curing envelope downwardly into such narrow grooves but frequently air remains in the grooves beneath the surface of the curing envelope due to the envelope's inability to conform to the tread design. The remaining air insulates the rubber at the bottom of the groove from the curing steam so that the parts of the tread stock and cushion gum beneath the grooves are not adequately heated or pressurized to effect a tight bond to the surface of the tire casing.
It has been suggested that incompressible lengths of material be placed in such grooves in order that the curing envelope will press upon that material and that material will press downwardly upon the cushion gum and tread stock material at the bases of the grooves. While this may press the tread stock against the tire casing more effectively, it still insulates that portion of the cushion gum from the heat. As a result, the adhesion of the pre-cured tread to the surface of the casing is not uniform across the width of the tread.
It is therefore the principal object of the instant invention to provide a pre-cured tread stock for recapping vehicle tires which is so designed as to enable the wings of the tread stock to be mechanically retained and squeezed tightly against the surface of the tire casing during the curing process.
It is another important object of the invention to provide a method for carrying out retreading or re-capping using the pre-cured tread stock of the invention by which method the assembly of the tread stock and cushion gum is held tightly against the surface of the tire casing during the curing process.